#define  _CRT_SECURE_NO_WARNINGS


 //Definition for singly-linked list.
 struct ListNode {
     int val;
     ListNode *next;
     ListNode(int x) : val(x), next(nullptr) {}
 };

class Solution {
public:
    bool hasCycle(ListNode* head) {
        ListNode* fast = head;
        ListNode* slow = head;
        while (fast && fast->next)
        {
            fast = fast->next->next;
            slow = slow->next;
            if (fast == slow) return true;
        }
        return false;
    }
};